Fig. 13.1A , cardiac cells are connected to each other through intercalated disks, which include a combination of mechanical junctions and electrical connections. The mechanical connections, which keep the cells from pulling apart when contracting, include the fascia adherens and desmosomes. Gap junctions between cardiac muscle cells, on the other hand, provide electrical connections between cells to allow propagation of the action potential throughout the heart. Thus the arrangement of cardiac muscle cells within the heart is said to form an electrical and mechanical syncytium that allows a single action potential (generated within the sinoatrial node) to pass throughout the heart so that the heart can contract in a synchronous, wave-like manner. Blood vessels course through the myocardium.
The basic organization of thick and thin filaments in cardiac muscle cells is comparable with that in skeletal muscle (see ). Electron microscopy reveals repeating light and dark bands that represent I bands and A bands, respectively (see
Fig. 13.1B
Fig.
12.3 ). Thus cardiac muscle is classified as a striated muscle. The Z line transects the I band and represents the point of attachment of the thin filaments. The region between two adjacent Z lines represents the sarcomere, which is the contractile unit of the muscle cell. The thin filaments are composed of actin, tropomyosin, and troponin and extend into the A band. The A band is composed of thick filaments, along with some overlap of thin filaments. The thick filaments are composed of myosin and extend from the center of the sarcomere toward the Z lines.
Myosin filaments are formed by a tail-to-tail association of myosin molecules in the center of the sarcomere, followed by a head-to-tail association as the thick filament extends toward the Z lines. Thus the myosin filament is polarized and poised for pulling the actin filaments toward the center of the sarcomere. A cross-section view of the sarcomere near the end of the A band shows that each thick filament is surrounded by six thin filaments, and each thin filament receives cross-bridge attachments from three thick filaments. This complex array of thick and thin filaments is characteristic of both cardiac and skeletal muscle and helps • Fig. 13.1 A, Photomicrograph of cardiac muscle cells (magnification, ×210). Intercalated disks at either end of a muscle cell are identified in the lower left portion of the micrograph. The intercalated disk physically connects adjacent myocytes and, because of the presence of gap junctions, electrically couples the cells as well so that the muscle functions as an electrical and mechanical syncytium. B, Schematic representation of the organization of a sarcomere within a cardiac muscle cell. (A, From Telser A. Elsevier’s Integrated Histology. St. Louis: Mosby; 2007. B, Redrawn from Fawcett D, McNutt NS. The ultrastructure of the cat myocardium. I. Ventricular papillary muscle. J Cell Biol. 1969;42:1-45.) stabilize the filaments during muscle contraction (see
Fig.
12.3B , for the hexagonal array of thick and thin filaments in the sarcomere of striated muscle).